Tandem Mass-Spectrometer Study of D3O+ and Solvated Derivatives

Abstract

The mass spectra of heavy water produced by electron impact on neutral gas at pressures up to 1 torr have been studied in a tandem mass spectrometer. The enthalpies of hydration of solvated D3O+ as well as the deuteron affinity of D2O were studied by measurement of kinetic-energy thresholds for the dissociation of the respective ions in collision with helium. Evidence was obtained for excited states in D3O+, D5O2+, D7O3+, D9O4+, etc., which survived for time periods in excess of 10−4 sec. Efforts to de-excite these ions with neutral D2O gas molecules in the high-pressure source were rarely successful, indicating difficulties in the interpretation of data based on the assumption of thermodynamic equilibrium in such ion sources. Values for the heat of association of two and three neutral heavy-water molecules were deduced from the threshold data. Relative cross sections for the attenuation of various ionic species were measured. The significant differences in the attenuation cross section point up problems in sampling steady-state concentrations of ions in a high-pressure ion source. The relative attenuation cross sections were used to test the validity of a point-charge–dipole interaction model for calculation of hydration enthalpies in the water systems. This model was shown to provide a very simple means of demonstrating an excellent correlation between experimentally determined enthalpies and cross-section data.